Until very recently, asbestos has been the standard, most widely used material in braided "compression" packings. The recent discovery of the potential carcinogenic effects of asbestos, as well as the fact that usable supplies of the raw material are naturally limited as well as not widely distributed geographically and, hence, subject to political restraints, has caused a wide search for alternative materials in the fluid sealing industry. In a patent issued to one of us (U.S. Pat. No. 3,306,155), it was disclosed that the substitute material is glass fiber, in combination with polytetrafluoroethylene (TFE) dispersion, forms a braided packing which is effective for a number of applications. However, glass fiber is attacked by alkalis and so cannot be used in such an environment. Also, it has a tendency to abrade on itself.
While each of the substitute materials on the market, such as glass fiber in combination with TFE dispersion, TFE fiber, graphite filament, etc., has substantial functional merit, they are considerably more expensive than the standard graphited asbestos packing which has been the predominant braided packing for many years. The search for an economical substitute, one which will be equivalent in cost to graphited asbestos packing, has not yet been successful.
Attempts have been made to incorporate graphite lubricant or other non-performance-limiting lubricants into a braided glass fiber structure. These attempts have hitherto yielded unsatisfactory results. Dry, flake graphite will not be retained by a braided glass fiber structure. When the graphite is mixed with an oil lubricant, the oil will seep out and not be retained; furthermore, the use of oil limits the effective operating temperature range of the packing. Also, the wet, dripping packing so constructed would create both housekeeping and safety problems. Adding graphite to a more coherent, waxy lubricant would help to retain the graphite in the structure. However, this mixture would fundamentally amount to a wax filler in which the graphite would have no significant value; furthermore the wax would limit the effective temperature of the packing to a very low range. An aqueous graphite dispersion also will not be retained in the braided glass fiber structure.
Experience derived from graphited asbestos packing offers no clear path to the development of a viable graphited glass fiber packing. The ordinary asbestos yarn of commercial grade or better used in the manufacture of braided packing contains up to 25% or even more of a cotton or rayon binder which readily absorbs and forms a reservoir of lubricant permitting the finished product to retain the lubricant within the braided structure indefinitely. Furthermore, the inherent structure of asbestos, containing multifarious voids as the result of uneven and very small and varied fibers which are incorporated into the yarn (as opposed to the smooth, regular filamentary nature of glass fiber), permits the retention of graphite-laden lubricants (or mica-laden, talc-laden or other particulate-laden mixtures) as well as flake graphite in dry, powdered form. Thus, braided asbestos packing has been easy and convenient to load with lubricants and the technique of doing so is well known.
Attempts to use the same technique on braided glass fiber have met with failure owing to the inherent characteristics of this fiber. Lately, attempts have been made to use more sophisticated, aqueous-based graphite-laden dispersions and inorganic lubricants such as molybdenum disulphide and titanium disulphide, for the same purpose. Such dispersions are available from Joseph Dixon Co., and, differing from the traditional mixture of graphite and oil, they do appreciably penetrate the glass fiber structure. However, two difficulties have been found with such materials. In the first place, they tend to wear off or, under pressure, blow out of a glass fiber braided structure more readily than from braided asbestos, since they are not held and protected by the same irregular fibrous structure nor are they suspended in a retained lubricant vehicle. Secondly, when such dispersions are applied and the packing dries, a hairy, brush-like surface emerges on the outer surface of the packing. Since a prime desideratum of any packing is a smooth, antifrictional surface, such a packing becomes highly suspect from a tactile point of view to the normal user who is accustomed to use "smoothness" of finish as one of the criteria for packing evaluation.
Wilkinson in U.S. Pat. No. 3,196,737 has described a gland-packing material consisting of asbestos cords and unsintered TFE cords braided together; unsintered TFE cords are usually produced by extruding coagulated TFE through a die. Unsintered TFE deforms so readily under stress that it can contribute nothing to the mechanical strength of such a packing, and so must be regarded essentially as a solid lubricant similar to dispersed TFE after drying, rather than as an organic fiber. Consequently, the packing of asbestos fiber or cord in combination with unsintered TFE cord has essentially the characteristics of a lubricated asbestos. Although relatively low in cost, the combination is carcinogenic, as are other asbestos-based packings. Accordingly, it would be highly desirable to develop a non-asbestos packing which enjoys the advantage of resilience contributed by suitable filaments and in which the solid lubricant is relatively inexpensive. The packing taught herein meets these objectives.